Electron-hole excitations in Mg₂Si and Mg₂Ge compounds

The recently implemented all-electron GW approximation shows that the conduction band quasiparticle energies of Mg₂Si and Mg₂Ge compounds are shifted by a constant energy Δ toward higher energies compared to the local-density approximation results whereas the valence bands remain unchanged. Including excitonic effects considerably improves the optical spectra peak positions and their intensities obtained within the random-phase approximation. The calculation predicts a low energy structure that is present in the experimental optical spectrum of Mg₂Ge but remains to be observed for Mg₂Si.